Spin-current switchable magnetic memory element and method of fabricating the memory element

ABSTRACT

A spin-current switchable magnetic memory element (and method of fabricating the memory element) includes a plurality of magnetic layers having a perpendicular magnetic anisotropy component, at least one of the plurality of magnetic layers including an alloy of a rare-earth metal and a transition metal, and at least one barrier layer formed adjacent to at least one of the plurality of magnetic layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation Application of U.S. patentapplication Ser. No. 10/990,401 filed on Nov. 18, 2004, which isincorporated by reference herein and from which the present Applicationclaims priority.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a spin-current switchable magneticmemory element and a method of fabricating the memory element, and moreparticularly, to a spin-current switchable magnetic memory elementincluding a plurality of magnetic layers, at least one of the pluralityof magnetic layers including an alloy of a rare-earth metal and atransition metal.

2. Description of the Related Art

A two-terminal, bi-stable resistor that is current-switchable can beused as a memory element. One class of such device is a spin-currentswitchable magnetic junction (e.g., a tunnel junction or a spin-valvejunction). The basic physics of these devices is verified in the pastfew years. The device holds the promise of being the next generationmagnetic memory element for scaling down to junction dimensions of 50 nmand below.

Conventional junction devices that have been experimentally demonstratedrequire too large a switching current—on the order of mid 10⁶ A/cm², andthe junction impedance is too low, only about 1 to 2Ω-μm². Industry-wideefforts are underway to reduce the amount of switching current required,and to increase the device impedance, for effective integration withback-end-of-line (BEOL) CMOS technology.

One method of reducing switching current is by introducing magneticmaterials that have a perpendicular magnetic anisotropy. This idea hasbeen quantitatively discussed with respect to related art methods.

A few combinations of materials choices have been previously discussedwith respect to related art methods. However, the previously discussedmaterials choices may tend to require sophisticated thin film synthesistechnology such as molecular beam epitaxy.

SUMMARY OF THE INVENTION

In view of the foregoing and other exemplary problems, disadvantages,and drawbacks of the aforementioned systems and methods, it is a purposeof the exemplary aspects of the present invention to provide aspin-current switchable magnetic memory element (e.g., a spin-currentinjection device) and method of manufacturing the memory element whichincludes materials that allow the memory element to be manufacturedwithout necessarily requiring sophisticated thin film synthesistechnology such as molecular beam epitaxy.

An exemplary aspect of the present invention includes a spin-currentswitchable magnetic memory element which includes a plurality ofmagnetic layers having a perpendicular magnetic anisotropy component, atleast one of the plurality of magnetic layers comprising an alloy of arare-earth metal and a transition metal, and at least one barrier layerformed adjacent to at least one of the plurality of magnetic layers. Thealloy may include, for example, a GdCo alloy, a TbFeCo alloy, etc.

The plurality of magnetic layers may include a first magnetic layer anda second magnetic layer, the at least one barrier layer being formedbetween the first and second magnetic layers. At least one of the firstand second magnetic layers may include a current-switchable magneticmoment.

Importantly, at least one of the plurality of magnetic layers may beformed by a sputter deposition process. Further, at least one of theplurality of magnetic layers may include a cobalt layer formed onplatinum, or a cobalt layer formed on gold. In addition, a thickness ofthe cobalt layer may be less than about 30 Å.

Further, at least one of the plurality of magnetic layers may include amagnetically free layer which can be rotated by a spin-currentinjection. In addition, the magnetic layer(s) including the alloy mayinclude a fixed magnetic layer which may have a sufficient thickness anda sufficient magnetic anisotropy to stay fixed during a current-inducedswitching process. In addition, the magnetic layer(s) the alloy mayinclude a fixed magnetic layer which provides a sufficient amount ofspin-polarized current.

Further, the at least one of the plurality of magnetic layers includingthe alloy may include a thickness of at least 100 Å.

The plurality of magnetic layers may include, for example, a firstmagnetically fixed layer, a magnetically free layer formed on the firstmagnetically fixed layer, and a second magnetically fixed layer formedon the magnetically free layer. The at least one barrier layer mayinclude, a first barrier layer formed between the first magneticallyfixed layer and the magnetically free layer, and a second barrier layerformed between the magnetically free layer and the second magneticallyfixed layer.

The magnetic memory element may also include a first lead formedadjacent to one of the plurality of magnetic layers, a second leadformed adjacent to another one of the plurality of magnetic layers, anda pillar formed between the first and second leads, the pillar includingthe at least one barrier layer and at least one of the plurality ofmagnetic layers. The at least one magnetic layer included in the pillarmay include the current-switchable magnetic moment. Specifically, themagnetic moment of the at least one magnetic layer included in thepillar may be switchable by an electrical current. In particular, themagnetic moment of the at least one magnetic layer included in thepillar may be switchable by an electrical current having a density of nomore than about 10⁶ A/cm².

Further, the barrier layer preserves spin information for an electriccurrent injected into the pillar and provides a resistance to thecurrent. In addition, at least one of the first and second leadsincludes a magnetic layer of the plurality of magnetic layers.

The pillar may include, for example, a lithographed pillar having adiameter of less than about 100 nm. The pillar may also have anelectrical resistance which depends on a magnetization direction of alower magnetic layer with respect to a magnetization direction of anupper layer.

The perpendicular magnetic anistropy component magnetic memory elementmay have a magnitude sufficient to at least substantially offset aneasy-plane demagnetization effect, such that a magnetic moment of one ofthe plurality of magnetic layers is either resting out of the film(e.g., layer) plane or can be rotated out of the film plane under spincurrent excitation.

Further, the at least one barrier layer may include a plurality ofbarrier layers which are alternately formed with the plurality ofmagnetic layers. In addition, the at least one barrier layer may includeat least one of an aluminum oxide layer, a magnesium oxide layer, adoped semiconductor layer, a non-magnetic metal layer and a SrTiO₃layer.

Another exemplary aspect of the present invention includes a magneticmemory element, including first and second leads, a pillar formedbetween the first and second leads, a plurality of magnetic layershaving a perpendicular magnetic anisotropy component, at least one ofthe plurality of magnetic layers comprising an alloy of a rare-earthmetal and a transition metal, and at least one barrier layer formedadjacent to at least one of the plurality of magnetic layers.

Another exemplary aspect of the present invention includes a magneticrandom access memory (MRAM) array including a plurality of magneticmagnetic memory elements according to at least one of the aspectsdiscussed above.

Another aspect of the present invention includes a method of fabricatinga magnetic memory element. The method includes providing a wafer havinga bottom electrode, forming a plurality of layers, such that interfacesbetween the plurality of layers are formed in situ, lithographicallydefining a pillar structure from the plurality of layers, and forming atop electrode on the pillar structure. The plurality of layers includesa plurality of magnetic layers having a perpendicular magneticanisotropy component, at least one of the plurality of magnetic layerscomprising an alloy of a rare-earth metal and a transition metal, and atleast one barrier layer formed adjacent to at least one of the pluralityof magnetic layers. The at least one barrier layer may include aplurality of barrier layers which are alternately formed with theplurality of magnetic layers.

With its unique and novel features, the present invention provides amagnetic memory element (e.g., a spin-current injection tunnelingdevice) which includes materials that allow the memory element to bemanufactured without necessarily requiring sophisticated thin filmsynthesis technology such as molecular beam epitaxy.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other exemplary purposes, features, aspects andadvantages will be better understood from the following detaileddescription of the exemplary embodiments of the invention with referenceto the drawings, in which:

FIG. 1A-1D illustrate a spin-current switchable magnetic memory element100, in accordance with an exemplary aspect of the present invention;

FIGS. 2A-2B illustrate a spin-current switchable magnetic memory element200, in accordance with an exemplary aspect of the present invention;

FIGS. 3A-3C a spin-current switchable magnetic memory element 300,according to an exemplary aspect of the present invention; and

FIG. 4 illustrates a method 400 of fabricating a spin-current switchablemagnetic memory element, according to an exemplary aspect of the presentinvention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS OF THE INVENTION

Referring now to the drawings, FIGS. 1A-1D, 2A-2B, 3A-3C and 4illustrate an exemplary aspect of the present invention. Specifically,these figures illustrate a magnetic memory element (e.g., acurrent-switchable two-terminal magnetic memory element) which may beincluded, for example, as part of a magnetic random access memory (MRAM)array.

In particular, as illustrated in FIG. 1A, an exemplary aspect of thepresent invention includes a spin-current switchable magnetic memoryelement 100 which includes a plurality of magnetic layers 121, 122having a perpendicular magnetic anisotropy component, at least one ofthe plurality of magnetic layers including an alloy of a rare-earthmetal and a transition metal. The memory element 100 further includes atleast one barrier layer 125 formed adjacent to at least one of theplurality of magnetic layers 121, 122 (e.g., between two of the magneticlayers).

Generally, the exemplary aspects of the present invention may includeone more sets of materials combinations (e.g., magnetic layer materialcombinations) which are superior in terms of ease in fabrication. Anidea of the present invention is to use two different types ofperpendicular anisotropy materials for the magnetically “fixed” layerand the magnetically “free” layer. The particular choices of materialsutilized by the present invention can be synthesized with industrystandard sputter deposition process and will not necessarily requireelaborate thin film processing such as molecular epitaxy, making theprocess readily adaptable to existing manufacturing environment.

An important idea of the present invention is to combine two differenttypes of magnetic thin films with perpendicular magnetic anisotropy in amagnetic junction (e.g., a single magnetic tunnel junction). Thecombination thus created satisfies the requirement for an efficientspin-current switchable device, and at the same time makes the deviceeasy to manufacture.

One type of magnetic film with perpendicular anisotropy is a cobalt film(e.g., an ultra-thin cobalt film) residing on a noble metal surface(e.g., PtCo or AuCo). The interface electronic structure is such that itgives the cobalt film a strong interface magnetic anisotropy that isperpendicular to the interface.

This class of films can be made with room temperature UHV sputterdeposition. The resulting polycrystalline film with preferential (111)close-pack surface is sufficient to induce such interface anisotropy.This interface anisotropy, however, can only support a very thin layerof cobalt (e.g., usually less than about 15 Å to 30 Å in thickness) forit to have perpendicular magnetization alignment to the interface (e.g.,surface). This type of magnetic thin film structure is good for use asthe magnetically “free” layer, which should be thin and can be rotatedby a spin-current injection.

However, the same type of layer (e.g., the same composition of layer),can not be used for the magnetic “free” layer and the magnetic “fixed”or reference, layer. The fixed layer (e.g., the reference layer) shouldhave enough magnetic anisotropy to stay fixed during the entirecurrent-induced switching process and thick enough to provide sufficientamount of spin-polarized current. That is why another type of magneticfilm with perpendicular anisotropy should be used for the magnetic fixedlayer.

The second type of magnetic film (e.g., the magnetic “fixed” layer) mayinclude, for example, rare-earth/transition metal alloys, such as GdCoalloy or TbFeCo alloy. These are actually microscopic ferrimagnets, withrelatively small total magnetic moment, and can be engineered to haveperpendicular magnetic anisotropy in their amorphous or polycrystallinethin film form. Some of these thin films have long been used forperpendicular magneto-optical recording.

An advantage of using these materials (e.g., rare-earth/transition metalalloys) as the “fixed” layer is that these materials can be maderelatively thick (e.g., about 100 Å or more or even several hundredangstroms or more), and still retain a perpendicular magneticanisotropy. The fact that the materials can be thick, in combinationwith the fact that they are made of rare-earth containing magneticmaterials which is dynamically very lossy (and hence less susceptible tospin-current induced magnetic excitation or switching), make thesematerials (e.g., rare-earth/transition metal alloys) and ideal candidatefor the “fixed” magnetic layer.

Moreover, the choice of this material set (e.g., rare-earth/transitionmetal alloys) makes it feasible to engineer junction stacks with boththe fixed and the free magnetic layers with perpendicular magneticanisotropy. It may also allow easy fabrication of a three-layer magneticjunction geometry, such as ∥Fixed top |separator |free |separator |Fixedbottom ∥. This could further reduce the amount of switching currentrequired to switch a junction device, and to increase the junctionimpedance.

One exemplary aspect of the present invention uses the followingcombination of thin film materials:Pt(or Au)|Co|Oxide Barrier (e.g., MgO)|Co_(x)Fe_(1-x)|TbFeCo|Pt∥

That is, in this exemplary aspect, a layer of Co is formed on a layer ofPt (or Au), an oxide barrier is formed on the layer of Co, and so forth.In this case, for example, the thickness of the Pt (or Au) layer may beat least about 50 Å, the thickness of the Co layer may be in a rangefrom about 10-30 Å, the oxide barrier may be in a range from about 5-30Å, the thickness of the Co_(x)Fe_(1-x) layer (e.g., Co₈₀Fe₂₀) may be ina range from about 5-30 Å, the thickness of the TbFeCo layer may be in arange from about 100-500 Å and the thickness of the Pt layer may be in arange from about 50-500 Å.

On the left side of the tunnel barrier, the interface of PtCo may beused for interface perpendicular magnetic anisotropy. Dieny's group hasshown this can be done with PtCo system in (e.g., see Monso et al, Appl.Phys. Left. 80, 4157 (2002)). Thereby, a free-layer with perpendicularmagnetic axis and switchable by spin-current can be created, and withswitching current density at or below 10⁶ A/cm².

On the right side of the tunnel barrier, the TbFeCo film may be used tocreate the magnetically “fixed” layer with perpendicular anisotropy. The15 Å of CoFe at the interface may be used to increase tunnelmagnetoresistance (MR). The CoFe|TbFeCo system has been shown tofunction as a magnetic tunnel electrode at the same time preserving itsperpendicular magnetic orientation by Nishimura et al. JAP 91, 5246(2002).

Using a similar concept, one may stack the layer sequences, and create athree-magnetic-layer structure, such as:∥TbFeCo|Co_(x)Fe_(1-x)|Tunnel Barrier (e.g., MgO)|Co|Au|Co_(x)Fe_(1-x)|TbFeCo|Pt∥,where the first separation layer is a tunnel barrier, and the second isa spin-valve structure. The tunnel barrier provides largemagneto-resistance and hence large voltage signal for read-out, whilethe spin-valve structure can further provide enhanced spin-polarizationfor the current.

That is, in this exemplary aspect, a layer of Co_(x)Fe_(1-x) is formedon a layer of TbFeCo, a tunnel barrier is formed on the layer ofCo_(x)Fe_(1-x), and so forth. In this case, for example, the thicknessof the first TbFeCo layer may be in a range from about 100-500 Å, thethickness of the first Co_(x)Fe_(1-x) (e.g., Co₈₀Fe₂₀) layer may be in arange from about 5-30 Å, the thickness of the tunnel barrier may be in arange from about 5-30 Å, the thickness of the Co layer may be in a rangefrom about 10-30 Å, the thickness of the Au layer may be in a range fromabout 10-200 Å, the thickness of the second Co_(x)Fe_(1-x) layer may bein a range from about 5-30 Å, the thickness of the second TbFeCo layermay be in a range from about 100-500 Å and the thickness of the Pt layeris not necessarily limited (e.g., may be a part of a lead) but may beabout 50-500 Å.

An alternative stack may include, for example:∥TbFeCo|Co_(x)Fe_(1-x)|Tunnel Barrier (e.g., MgO) |Co|Au|Tunnel Barrier(e.g., MgO)|TbFeCo|Pt∥

That is, in this exemplary aspect, a layer of Co_(x)Fe_(1-x) is formedon a layer of TbFeCo, a tunnel barrier is formed on the layer ofCo_(x)Fe_(1-x), and so forth. In this case, for example, the thicknessof the first TbFeCo layer may be in a range from about 100-500 Å, thethickness of the first Co_(x)Fe_(1-x) (e.g., Co₈₀Fe₂₀) layer may be in arange from about 5-30 Å, the thickness of the tunnel barrier may be in arange from about 10-30 Å, the thickness of the Co layer may be in arange from about 10-30 Å, the thickness of the Au layer may be in arange from about 0-200 Å, the thickness of the second tunnel barrierlayer may be in a range from about 10-30 Å, the thickness of the secondTbFeCo layer may be in a range from about 100-500 Å and the thickness ofthe Pt layer is not necessarily limited (e.g., may be a part of a lead)but may be about 50-500 Å.

In this case, the first (left-side) tunnel barrier may givemagnetoresistance contrast for read-out, whereas the right side tunnelbarrier may function primarily as a supplier of spin-polarized currentand not necessarily provide much magnetoresistance (which could beadvantageous in some magnetic arrangements to avoid the cancellationeffect of magnetoresistance read-out).

Referring again to FIGS. 1A-1D, 2A-2B, 3A-3C and 4, the exemplaryaspects of the present invention make use of a perpendicular componentof magnetic anisotropy (e.g., a perpendicular magnetic anisotropycomponent) for the creation of a magnetic state that is more favorablefor low-current switching. The perpendicular anisotropy component maycounter the demagnetization field of a magnetic layer (e.g., theswitching, or “free” layer, or the fixed, or “pinned” reference magneticlayers, or both) of the magnetic layers that form the thin filmswitching element), making the magnetic anisotropy which is useful formemory functions the only significant energy barrier that a spin-currentswitch has to overcome.

As illustrated in FIG. 1A, the barrier layer 125 may include a tunnelingbarrier layer which is formed between two magnetic layers 121, 122. Thatis, the inventive spin-current switchable magnetic memory element mayinclude a magnetic tunneling junction.

As illustrated in FIG. 1B, the spin-current switchable magnetic memoryelement 100 may be formed on a substrate 110 (e.g., semiconductorsubstrate). Further, the spin-current switchable magnetic memory element100 may include a first lead 130 (e.g., a bottom electrode), and asecond lead 140 (e.g., top electrode).

Further, at least one of the plurality of magnetic layers 121, 122 maybe included as part of a pillar (e.g., a thin film stack having lateraldimensions on the order of 100 nm) 150 formed between the first andsecond leads 130, 140. The pillar may have, for example, an oblongcross-section, but the pillar may have a cross-section of other shapesand is not necessarily limited to any particular cross-section.

For example, as illustrated in FIG. 1C, all of the magnetic layers 121,122 may be included in the pillar 150. However, some (e.g., all but one)of the plurality of magnetic layers may be formed outside of the pillar.For example, as shown in FIG. 1D, magnetic layer 122 is formed in thepillar 150, but magnetic layer 121 is not formed in the pillar. Further,a magnetic layer not included in the pillar 150 may be formed as part ofthe first or second leads 130, 140. It should also be noted that thebarrier layer 125 may or may not be formed in the pillar.

In one exemplary aspect (e.g., illustrated in FIG. 1B), the first lead(e.g., bottom contact electrode) 130 may be formed between the substrate110 and the pillar 150, and the second lead 140 (e.g., top contactelectrode) may be formed on a top surface of the plurality of layers andopposite to the bottom contact electrode. The space between bottom lead(element 130) and top lead (element 140) can be filled with aninsulating material (e.g., silicon dioxide). An electrical current(e.g., a current having a density of no more than about 10⁶ A/cm²)flowing between the first and second leads 130, 140 via pillar 150 maycause a change in the magnetic moment (e.g., magnetization direction) ofone of the plurality of magnetic layers (e.g., magnetic layer 122 inFIGS. 1B-1D).

An important concept of the present invention is to utilize theperpendicular magnetic anisotropy component observed in some magneticthin films to counter-balance the strong demagnetization effect 4πM_(s),thus removing the main barrier for current-induced magnetic reversal,and reduce the switching current threshold.

A quantitative relationship may be established, both theoretically andwith some recent experimental verification, that a threshold current forspin-current induced switching can be expressed as follows (when4πM_(s)≧H_(p)):I _(c)=(1/η)(2e/

)α(a ² l _(m) M _(s))[H _(k) +H _(a)+(4πM _(s) −H _(p))/2]where η is the spin-polarization factor of the current, e is electroncharge,

=h/2π is the normalized Planck constant, α is the magnetic dampingcoefficient, l_(m) is the film thickness for the switching elementlayer, a² is the film area (e.g., lateral size squared), M_(s) is thesaturation magnetization of the switching element layer, H_(k) is theuniaxial anisotropy field of the switching element layer in the filmplane, and Hp is the perpendicular anisotropy field of the switchingelement layer induced either at the interface (such as for the case ofCo—Pt interface) or through epitaxy with its strain field or intrinsiccrystalline anisotropy of the material.

In addition, the term (a²l_(m)M_(s))(H_(k)) represents the uniaxialanistropy energy of the switching element (e.g., for a blockingtemperature T_(b) which reflects the thermal-stability limit forinformation storage), 4πM_(s) is the easy-plane shape anistropy andH_(a) is the applied field. Further explanation for materialperpendicular anisotropy field H_(p) is given below.

When H_(p) is larger than 4πM_(s), the magnetic moment prefers to orientitself perpendicular to the surface of the film, and the differenceH_(p)−4πM_(s) becomes a uniaxial anisotropy term as well. If theadditional uniaxial anisotropy H_(k) is also designed to beperpendicular, one has the ideal situation ofI _(c)=(1/η)(2e/

)α(a ² l _(m) M _(s))[(H _(k)+(H _(p)−4πM _(s))/2)+H _(a)]where all terms of anisotropy energy are uniaxial that contributes tothe nanomagnet's thermal stability. In this case one wastes no currenton overcoming any easy-plane anisotropy which does not contribute tothermal stability.

To implement such a perpendicular magnetic moment arrangement, one hasto not only have the “free” layer nanomagnet's magnetic moment becomeperpendicular, it is also necessary for the “fixed” or “reference”magnetic layer to have magnetic moment perpendicular to the filmsurface. This requires the engineering of relatively thick magneticfilms to have perpendicular magnetic anisotropy. This requirement can besatisfied by using the rare-earth-transition metal alloy systems such asGdFeCo prepared using sputter deposition.

For most magnetic thin films of interest for magnetic memoryapplications, the demagnetization term 4πM_(s) would be large comparedto H_(k). For cobalt, for example, the term is on the order of 16,000Oe, whereas H_(k) is usually less than 1,000 Oe. Ordinarily, the 4πM_(s)term is the main factor in controlling the switching current. Thedemagnetization energy is an easy-plane anisotropy, which is aconsequence of the flat geometry of a thin film nanomagnet.

The exemplary aspects of the present invention use the additionalmaterials and/or interface perpendicular magnetic anisotropy energy(whose effect is represented by H_(p) in the above formula), as a meansto counter this force. Specifically, the present invention may reducethe combined perpendicular anisotropy to a value (e.g., a minimum value)that is convenient for a spin-current induced switch.

Specifically, the exemplary aspects of the present invention may utilizetwo classes of possible mechanisms for perpendicular magneticanisotropy. One class originates from interface electronic interaction,the other class from bulk structural (e.g., strain) modulation in theplurality of magnetic layers (e.g., strain in a thin film nanomagnet).

A specific example belonging to the first class of mechanisms is theinterface-induced perpendicular magnetic anisotropy in thin films (e.g.,cobalt-gold films). It has been demonstrated experimentally thatultra-thin Pt/Co/Pt and Au/Co/Au films exhibit perpendicular anisotropylarge enough to completely overcome the thin film demagnetization fieldof cobalt. It has been further demonstrated experimentally that one canplace two layers of such materials adjacent to each other with differentperpendicular switching field strength.

Thus, referring again to FIGS. 1A-1D, the spin-current switchablemagnetic memory element may include two magnetic layers 121, 122separated by a barrier layer 125. An important aspect of the presentinvention is that at least one of the magnetic layers (e.g., magneticlayer 121) includes an alloy of a rare-earth metal and a transitionmetal, (e.g., magnetic layer 122).

Thus, for example, magnetic layer 121 may be free magnetic layer andinclude, for example, a cobalt layer formed on gold or platinum.Magnetic layer 122, on the other hand, may be a fixed magnetic layer andinclude, for example, an alloy of a rare-earth metal and a transitionmetal (e.g., a GdCo alloy, TbFeCo alloy, etc.). Further, the barrierlayer (e.g., tunneling barrier) 125 may include, for example, aluminumoxide or magnesium oxide.

As illustrated in FIGS. 1B-1F, the barrier layer 125 and at least one ofthe magnetic layers 121, 122 may be included in the lithographed pillar150 (e.g., an elongated cylinder-shaped pillar having a lateral sizeless than about 100 nm). The magnetization (e.g., magnetic moment) ofone magnetic layer (e.g., layer 122) may have a fixed orientation,whereas another magnetic layer (e.g., layer 121) may have a switchablemagnetization the direction which represents the information state.

In particular, a perpendicular magnetic anistropy may be included in themagnetic layer 122 which may have a magnitude sufficient to offset theeasy-plane demagnetization effect 4πM_(s) in magnetic layer 122. Thishelps to reduce the amount of current needed to change the magnetizationdirection of the magnetic layer 122. Further, incorporation of thebarrier layer (e.g., tunneling barrier layer) 125 adjacent to magneticlayer 122 suitable for spin-transfer switching helps to provide apractical signal voltage swing corresponding to the two differentmagnetic alignment states the adjacent magnetic layers have (e.g.parallel and antiparallel).

The magnetic layers of the spin-current switchable magnetic memoryelement may include a magnetic layer formed on a non-magnetic metallayer (e.g., cobalt on gold, cobalt on platinum, etc.). In this case, aperpendicular magnetic anisotropy component may be provided at aninterface between layers (e.g., between a magnetic layer and anon-magnetic layer) in the magnetic layer.

For example, FIG. 2A illustrates a magnetic memory element 200 havingfirst and second leads 130, 140, and a first magnetic layer 121including a layer of cobalt formed on a layer of platinum, a barrierlayer (e.g., tunneling barrier layer) 125 formed on the first magneticlayer 121, and a second magnetic layer 122 including an alloy of arare-earth metal and a transition metal (e.g., a GdCo alloy, TbFeCoalloy, etc.) which is formed on the barrier layer 125.

FIG. 2B illustrates another aspect in which the magnetic memory element200 having first and second leads 130, 140, and a first magnetic layer121 including a layer of cobalt formed on a layer of gold, a barrierlayer (e.g., tunneling barrier layer) 125 formed on the first magneticlayer 121, and a second magnetic layer 122 including an alloy of arare-earth metal and a transition metal (e.g., a GdCo alloy, TbFeCoalloy, etc.) which is formed on the barrier layer 125.

Further, as illustrated in FIGS. 1A-1D, the magnetic layers 121, 122 maybe included as part of the pillar 150 or lead 130, 140.

Although the exemplary aspects illustrated in FIGS. 1A-1D and 2A-2Billustrate a memory element having two magnetic layers 121, 122, it ispossible for the memory element of the present invention to include anynumber of magnetic layers 121 (e.g., free magnetic layers) and magneticlayers 122 (e.g., fixed magnetic layers)

For example, as illustrated in FIG. 3A, the memory element may include afirst and second leads 330, 340, and a pillar 350 which includesmagnetic layers 321 formed adjacent to the leads 330, 340, barrierlayers 325 formed adjacent to the magnetic layers 321, and a secondmagnetic layer 322 formed between the barrier layers 325.

FIG. 3B illustrates a magnetic memory element which is similar to thatin FIG. 3A. However, the magnetic memory element of FIG. 3B does notinclude the magnetic layers 321 as part of the pillar 350. Instead, themagnetic layers 321 are formed as part of the first and second leads330, 340.

FIG. 3C illustrates another exemplary aspect of the present invention.Specifically, the memory element 300 may include a fixed bottom magneticlayer 322, a separator layer (e.g., a tunneling barrier layer) 325formed on layer 322, a free magnetic layer 321, another separator layer(e.g., tunneling barrier layer) 325, and a fixed top magnetic layer 322.

It should be noted that in any of the exemplary aspects of the presentinvention (e.g., FIGS. 3A-3C), the barrier layers and magnetic layersmay include different materials. Thus, for example, one of the barrierlayers 325 in FIG. 3A may include aluminum oxide, whereas another one ofthe layers 325 may include magnesium oxide. Further, one of the magneticlayers 321 may include a Co layer formed on gold, whereas another one ofthe magnetic layers 321 may include a Co layer formed on platinum.Further, one of the magnetic layers 322 may include a GdCo alloy,whereas another one of the magnetic layers 322 may include a TbFeCoalloy, and so on.

In all exemplary structures discussed above, the perpendicularanisotropy component introduced through either interface or bulk straincan, but does not need to, completely overcome theshape-anisotropy-induced easy-plane magnetic anisotropy. That is, H_(p)needs to be close to but does not have to exceed, 4πM_(s) WhenH_(p)>4πM_(s), the nanomagnet layer responsible for switching (e.g., the“free” layer) will have its stable magnetization direction perpendicularto the thin film surface, either pointing up or down, representing the 0and 1 state of information.

In this case, the fixed, or reference layers of magnetic thin film(s)should be engineered to have its (their) magnetization resting in theperpendicular direction as well so as to provide the correct referencedirection, both for writing and for reading the magnetic bit. Thisgeometry may have superior magnetic stability over arrangements wherethe thin film magnetization stays within the thin film plane.

The inventors have previously pointed out that it is often non-trivialto engineer more than one magnetic thin film in the stack to have aperpendicular magnetic anisotropy component strong enough to overcomethe demagnetization. This is particularly so if the perpendicularanisotropy component originates from a strained thin film state.

If such is the case, it may be simpler and easier to engineer theperpendicular anisotropy component in the “free” layer to be just belowthat of the demagnetization field 4πM_(s). That is, to haveH_(p)≲4πM_(s), so that 4πM_(s)−H_(p)˜H_(k). This way, the “free” layer'smagnetization will still rest within the thin film plane, but it will beallowed to rotate out of the thin film plane upon spin-currentexcitation.

Since the resting position of the “free” layer remains within the thinfilm plane, the reference magnetic layer also need only have itsmagnetization resting within the thin film plane. This way, one is ableto avoid the difficult task of trying to engineer a structure where thefixed magnetic layer would also have to have its magnetization restperpendicular to the film surface. This will significantly lower thecomplexity of device materials engineering.

With the newly disclosed materials combination incorporating therare-earth-transition metal ferrimagnetic alloys such as GdFeCo orTbFeCo, it is possible to engineer the magnetic stack with all magneticfilms having a true perpendicular magnetic state. This can now beaccomplished because the thicker magnetic “reference” layer(s) can beprepared by using the ferrimagnetic alloys with sputter deposition.Under optimized deposition conditions these ferrimagnetic alloy thinfilms can be made with net-perpendicular magnetic anisotropy with filmthicknesses well above 100 Å. Furthermore, these ferrimagnetic alloythin films thus prepared have sufficient perpendicular magneticanisotropy and strong enough interface exchange coupling with otherferromagnetic films such as Co_(x)Fe_(1-x) (with x ranging between 0and 1) to force a perpendicular magnetic moment state in suchferromagnetic thin films (typically less than 30 Å or so) in directcontact with the ferrimagnet with an in-situ prepared interface. Anexample of such a structure is the |Co_(x)Fe_(1-x)|TbFeCo| bilayer asdiscussed earlier, and as has been experimentally demonstrated byNishimura's group (see Nishimura et al. J. Appl. Phys. 91, 5246 (2002)).

It is important to note that in any of the above-discussed aspects ofthe present invention (e.g., FIGS. 1A-3C), the incorporation of thetunneling barrier into the plurality of layers (e.g., pillar) suitablefor spin-transfer switching helps to provide a practical signal voltage.

Furthermore, a two-terminal bi-stable magnetic switch with relativelyhigh impedance and signal level is, from a circuit's perspective, verysimilar to other types of memory elements (e.g., ovonic unified memory(OUM) type, perovskite resistive memory, etc.) that are currently beingdeveloped. The present invention has all the advantages of speed andnonvolatility MRAM has to offer, and at the same time is compatible withthe circuit architecture of the other types of two-terminalresistive-switching memories, making it a much a broad-based memoryelement technology for future generations of MRAM and for systemintegration.

As noted above, the fixed magnetic layer (e.g., an alloy of a rare-earthmetal and a transition metal) may be formed to have a relatively largethickness (e.g., about 100 angstroms or more) compared to the thicknessof the free magnetic layer (e.g., cobalt on gold, cobalt on platinum,etc.) (e.g., less than about 30 angstroms). By tuning the thicknessesand materials of the magnetic layers, the present invention is able toobtain different perpendicular anisotropy fields for the magnetic layersadjacent to the tunneling barrier layer (e.g., the top magnetic layerand the bottom magnetic layer). This can give a unique switchingthreshold current for the free magnetic layers to be switchable bycurrent injection across the tunneling barrier layer, as well as for thefine-tuning of magnetic stability for the switching device.

Another exemplary aspect of the present invention includes a method offabricating a spin-current switchable magnetic memory element. Themethod may include, for example, forming a plurality of magnetic layershaving a perpendicular magnetic anisotropy component, at least one ofthe plurality of magnetic layers, at least one of the plurality ofmagnetic layers including an alloy of a rare-earth metal and atransition metal, s, and at least one barrier layer formed adjacent toat least one of the plurality of magnetic layers. The inventive methodmay include all of the features and functions described above withrespect to the inventive spin-current switchable magnetic memoryelement.

FIG. 4 illustrates an exemplary aspect of the method of fabricating aspin-current switchable magnetic memory element according to theexemplary aspects of the present invention. As illustrated in FIG. 4,the method 400 of fabricating a spin-current switchable magnetic memoryelement includes providing (410) a wafer having a bottom electrodeformed thereon, forming (420) a plurality of layers, the plurality oflayers including a plurality of magnetic layers, at least one of theplurality of magnetic layers, at least one of the plurality of magneticlayers including an alloy of a rare-earth metal and a transition metal,and at least one barrier layer formed adjacent to said plurality ofmagnetic layers, lithographically defining (430) a pillar structure fromthe plurality of layers, and forming (440) a top electrode on saidpillar structure.

It should be noted that the barrier layer(s) may be formed during theformation of the plurality of magnetic layers. For example, the methodmay include forming a first magnetic layer, forming a barrier layer onthe first magnetic layer, and forming a second magnetic layer on thebarrier layer. Another barrier layer may then be formed on the secondmagnetic layer, and so on.

Further, it is important to note that the magnetic layers in the claimedinvention (e.g., the at least one magnetic layer including an alloy of arare-earth metal and a transition metal) may be formed by a sputterdeposition process. That is, the materials (e.g., in the magneticlayers) in the present invention may be selected such that the method400 does not require the use of sophisticated thin film synthesistechnology such as molecular beam epitaxy.

With its unique and novel features, the present invention provides amagnetic memory element (e.g., a spin-current injection tunnelingdevice) which includes materials that allow the memory element to bemanufactured without necessarily requiring sophisticated thin filmsynthesis technology such as molecular beam epitaxy.

While the invention has been described in terms of one or more exemplaryembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims. Specifically, one of ordinary skill in the art willunderstand that the drawings herein are meant to be illustrative, andthe design of the inventive assembly is not limited to that disclosedherein but may be modified within the spirit and scope of the presentinvention.

It should be noted that the term “a plurality of layers having aperpendicular magnetic anisotropy component” should be broadly construedto mean that at least one layer in the plurality of layers has acomponent of perpendicular magnetic anisotropy. Further, the term“perpendicular magnetic anisotropy component” should be broadlyconstrued to mean “at least some component” of perpendicular magneticanisotropy. That is, a layer (e.g., a layer having a perpendicularmagnetic anisotropy component) does not need to exhibit a total magneticanisotropy that is perpendicular, but need only to have itsshape-induced easy-plane anisotropy reduced (e.g., significantlyreduced) by the said component of perpendicular anisotropy.

Further, the term “fixed” layer as used herein should be broadlyconstrued to include a layer having a net perpendicular anisotropy(e.g., a layer having a magnetic moment resting in a position that isout of the plane of the film surface (e.g., perpendicular to the filmsurface)). The magnetic moment of the “fixed” layer does not need to betotally perpendicular for the present invention to work, but thisfeature is enabled by the present invention (e.g., by the particularcombination of materials of the present invention), and is a significantfeature of the present invention.

In addition, it should be noted that terms such as “an X layer” and “alayer of X” (e.g., where X is cobalt, gold, etc.) included herein may beconstrued to mean a layer including X. That is, for example, the term “acobalt layer” and “a layer of cobalt” may be construed to mean a layerincluding cobalt.

Further, Applicant's intent is to encompass the equivalents of all claimelements, and no amendment to any claim the present application shouldbe construed as a disclaimer of any interest in or right to anequivalent of any element or feature of the amended claim.

1. A spin-current switchable magnetic memory element, comprising: aplurality of magnetic layers having a perpendicular magnetic anisotropycomponent, said plurality of magnetic layers comprising a first magneticlayer comprising an alloy of a rare-earth metal and a transition metal,and a second magnetic layer including a material which is different thansaid alloy; and a barrier layer formed adjacent to said plurality ofmagnetic layers.
 2. The magnetic memory element according to claim 1,wherein said perpendicular magnetic anistropy component has a magnitudesufficient to at least substantially offset an easy-planedemagnetization effect.
 3. The magnetic memory element according toclaim 1, wherein a magnetic moment of said second magnetic layer is oneof resting out of the layer plane and rotatable out of the layer planeunder spin current excitation.
 4. The magnetic memory element accordingto claim 1, wherein said barrier layer is formed between said first andsecond magnetic layers.
 5. The magnetic memory element according toclaim 1, wherein said second magnetic layer comprises acurrent-switchable magnetic moment.
 6. The magnetic memory elementaccording to claim 1, wherein at least one of said plurality of magneticlayers is formed by sputter deposition.
 7. The magnetic memory elementaccording to claim 1, wherein said second magnetic layer comprises acobalt layer formed on platinum.
 8. The magnetic memory elementaccording to claim 1, wherein said second magnetic layer comprises acobalt layer formed on gold.
 9. The magnetic memory element according toclaim 1, wherein said second magnetic layer comprises a magneticallyfree layer which can be rotated by a spin-current injection.
 10. Themagnetic memory element according to claim 1, wherein said firstmagnetic layer comprises a fixed magnetic layer having a sufficientthickness and a sufficient magnetic anisotropy to stay fixed during acurrent-induced switching process.
 11. The magnetic memory elementaccording to claim 10, wherein said fixed magnetic layer provides asufficient amount of spin-polarized current for switching a magneticmoment of said second magnetic layer.
 10. The magnetic memory elementaccording to claim 1, wherein said alloy comprises a GdCo alloy.
 11. Themagnetic memory element according to claim 1, wherein said alloycomprises a TbFeCo alloy.
 12. The magnetic memory element according toclaim 1, wherein said first magnetic layer comprises a thickness of morethan 100 Å and said second magnetic layer comprises a thickness of lessthan about 30 Å.
 14. The magnetic memory element according to claim 1,further comprising: a first lead formed adjacent to one of saidplurality of magnetic layers; a second lead formed adjacent to anotherone of said plurality of magnetic layers; and a pillar formed betweensaid first and second leads, said pillar including said barrier layerand at least one of said plurality of magnetic layers.
 15. The magneticmemory element according to claim 14, wherein a magnetic moment of saidat least one magnetic layer included in said pillar is switchable by anelectrical current having a density of no more than about 10⁶ A/cm². 16.The magnetic memory element according to claim 14, wherein said barrierlayer preserves spin information for an electric current injected intosaid pillar and provides a resistance to said current.
 17. The magneticmemory element according to claim 14, wherein said pillar comprises alithographed pillar having a diameter of less than about 100 nm.
 18. Themagnetic memory element according to claim 1, wherein said secondmagnetic layer comprises a cobalt layer having a thickness of less thanabout 30 Å.
 19. A method of fabricating a magnetic memory element, saidmethod comprising: forming a plurality of magnetic layers having aperpendicular magnetic anisotropy component, said plurality of magneticlayers comprising a first magnetic layer comprising an alloy of arare-earth metal and a transition metal, and a second magnetic layerincluding a material which is different than said alloy; and forming abarrier layer adjacent to said plurality of magnetic layers.
 20. Themethod of fabricating a magnetic memory element according to claim 19,wherein said perpendicular magnetic anistropy component has a magnitudesufficient to at least substantially offset an easy-planedemagnetization effect.